Wind turbines are receiving increasingly more attention as an environmentally safe and relatively inexpensive alternative energy sources. Wind turbines do not emit greenhouse gases (GHGs), and therefore, do not contribute to global warming. With the growing interest in wind generated electricity, considerable efforts have been made to develop wind turbines that are reliable and efficient.
Typically, wind turbines are used to convert the kinetic energy in the wind into mechanical power. This mechanical power may be used for specific tasks (such as pumping water) or a generator may convert this mechanical power (i.e., the rotation of a shaft) into electricity. A wind turbine usually includes an aerodynamic mechanism (e.g., blades) for converting the movement of air into a mechanical motion (e.g., rotation), which is then converted with a generator into electrical power. Power output from the generator is proportional to the cube of the wind speed. As wind speed doubles, the capacity of wind generators increases almost eightfold.
The majority of commercially available wind turbines utilize multi-stage geared drive trains to connect the turbine blades to the electrical generators. The wind turns the turbine blades, which spin a low speed shaft. The low speed shaft is coupled to an input shaft of a gearbox, which has a higher speed output shaft connected to a generator. Thus, the geared drive aims to increase the velocity of the mechanical motion.
The majority of geared drive trains in existing wind turbines of ratings >1 MW utilize multiple gear stages to achieve gear ratios ranging from about 1:70 up to about 1:110. The three stages typically comprise a simple planetary or epicylic first stage, followed by two parallel offset stages (bull-gear+pinion gears) or a second simple planetary stage followed by a parallel offset stage. The high gear ratio enables a generator that is substantially smaller and lower cost than the gearbox. The relatively high-speed of the generator forces the generator to have an aspect ratio that is longer than it is wide, with radial-vented cooling. The high-speed output shaft of the gearbox is generally not concentric with the low-speed input shaft of the gearbox. For these reasons, the generator is mounted separately and spaced from the gearbox on a mainframe (also commonly called a bedframe or bedplate). Power is transferred from the gearbox to the generator via a flexible “high-speed” shaft coupling. This arrangement forces the gearbox and generator to be physically distanced from each other, as well as requires both the output shaft of the gearbox and the input shaft of the generator to be separately supported by gearbox bearings and generator bearings, respectively.
A more optimal configuration of a geared drive train is therefore desirable by the wind industry to provide increased reliability and availability, reduced cost, reduced mass and size, and increased efficiency.